Hot box design with a multi-stream heat exchanger and single air control

ABSTRACT

A fuel cell stack module includes a plurality of fuel cell stacks, an anode tail gas oxidizer (ATO) which is located in a heat transfer relationship with the plurality of fuel cell stacks, a base supporting the plurality of fuel cell stacks and the ATO, and at least one heat exchanger located in the base. An ATO exhaust stream and an anode exhaust stream from the fuel cell stacks heat the stack fuel and air inlet streams in a multi-stream heat exchanger.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application claims benefit of U.S. provisional application60/935,092, filed Jul. 26, 2007, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of fuel cellsystems and more particularly to a fuel cell system containing amulti-stream heat exchanger and method of operating same.

Fuel cells are electrochemical devices which can convert energy storedin fuels to electrical energy with high efficiencies. High temperaturefuel cells include solid oxide and molten carbonate fuel cells. Thesefuel cells may operate using hydrogen and/or hydrocarbon fuels. Thereare classes of fuel cells, such as the solid oxide regenerative fuelcells, that also allow reversed operation, such that oxidized fuel canbe reduced back to unoxidized fuel using electrical energy as an input.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a three dimensional cut away view of a fuel cell module of anembodiment of the invention with a shell removed. FIG. 1B is a schematicside cross sectional view of the module of FIG. 1A. FIG. 1C is a topview of the module of FIG. 1A. FIGS. 1D and 1E are top views of themodule according to alternative embodiments of the invention.

FIGS. 2A, 2B and 5 are schematic diagrams of the components and fluidflow directions of fuel cell systems of embodiments of the invention.

FIG. 3 is a computer simulation of a plot heat exchanger heat dutyversus temperature for a heat exchanger according to an embodiment ofthe present invention.

FIG. 4 is schematic diagram of the zones and fluid flow directions ofthe heat exchanger according to an embodiment of the present invention.

FIGS. 6 and 7 are three dimensional cut-away views of two types ofmulti-stream plate heat exchangers that may be used in embodiments ofthe present invention.

FIG. 8 is a schematic three dimensional view of a modular fuel cellsystem according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The first embodiment of the invention provides a fuel cell stack module1 which is illustrated in FIGS. 1A, 1B and 1C. The module 1 contains abase 3, which comprises a chamber 5 and a base plate 7 above the chamber5 which provides an upper surface of the base 3. The base 3 may have acylindrical shape, with a flat upper surface and a circular crosssection, as shown in FIGS. 1A and 1C. However, the base 3 may have anyother suitable shape, such as a square, rectangular, polygonal, oval orirregular cross section. The base plate 7 may comprise a separatecomponent which is attached to the chamber 5 or the base 3 may comprisea unitary component in which the chamber 5 comprises its interior volumeand the base plate 7 comprises its upper surface. As will be describedbelow, one or more heat exchangers 13 can be located inside the chamber5.

As shown in FIGS. 1A-1C, each fuel cell stack module 1 includes at leastone fuel cell stack column 9 (which will be referred herein as a “stack”for simplicity) and an outer shell 11. The shell 11 can have anysuitable shape, such as a dome, a covered cylinder (including a cylinderwith a flat top cover or a cylinder with a dome shaped cover (whichhelps to reduce thermal stress)), a cube or a three dimensionalrectangle, for covering the stack(s) 9. The shell 11 is shown in FIG. 1Band its location from the top is shown as a dashed line in FIGS. 1C-1E.For example, two or more stacks 9, such as four to twelve stacks 9 maybe located under the shell 11. The stacks 9 are preferably stackedvertically under each shell 11. If desired, the vertically stacked fuelcell stacks 9 may be provided in a cascade configuration, where the fuelexhaust stream from one stack is used as the inlet fuel stream for anadjacent stack, such as, for example, a cascade to and from modules inthe same column.

The stacks 9 may comprise any suitable fuel cells. For example, the fuelcells may comprise solid oxide fuel cells having a ceramic oxideelectrolyte. Other fuel cell types, such as PEM, molten carbonate,phosphoric acid, etc. may also be used. The stacks 9 may compriseexternally and/or internally manifolded stacks. For example, the stacksmay be internally manifolded for fuel and air with fuel and air risersextending through openings in the fuel cell layers and/or in theinterconnect plates between the fuel cells. Alternatively, as shown inFIGS. 1B and 1C, the fuel cells may be internally manifolded for fueland externally manifolded for air, where only the fuel inlet and exhaustrisers extend through openings in the fuel cell layers and/or in theinterconnect plates between the fuel cells. The fuel cells may have across flow (where air and fuel flow roughly perpendicular to each otheron opposite sides of the electrolyte in each fuel cell), counter flowparallel (where air and fuel flow roughly parallel to each other but inopposite directions on opposite sides of the electrolyte in each fuelcell) or co-flow parallel (where air and fuel flow roughly parallel toeach other in the same direction on opposite sides of the electrolyte ineach fuel cell) configuration. Each stack 9 may have one fuel inlet andoutlet, as will be described in more detail below. However, if desired,each stack 9 may have several fuel inlets and outlets along its height.In that case, each stack 9 contains plural sub-stack units (i.e., eachstack column 9 contains separate sub-stacks).

As shown in FIGS. 1C, 1D and 1E, the plurality of angularly spaced fuelcell stacks 9 are arranged to form an annular array (i.e., a ring-shapedstructure) about a central axis of the module. It should be noted thatthe term “annular array” is not limited to an array having a circularperimeter, which is shown in FIG. 1D. For example, the array may have ahexagonal or rectangular (square) perimeter, as shown in FIGS. 1C and1E, respectively, or elliptical perimeter, which would provide anarrower system, which could then more easily fit in a standard shippingcontainer. The fuel cell stacks 9 have a stacking direction extendingparallel to the central axis of the module 1. Preferably, but notnecessarily each of the stacks 9 has a rectangular cross section. Thestacks 9 are isolated from each other using ceramic or other insulatingspacers. While stacks 9 arranged as an annular array are preferred, anyother stack 9 layout which would fit within the shell 11 may be used,such as an arc layout (i.e., a portion of a ring) or a grid layout (e.g.20 stacks, 4 rows by 5 columns) for example.

The shell 11 may have any suitable configuration. For example, the shell11 may have a cylindrical configuration. However, the shell 11 may havea polygonal or oval horizontal cross section and/or it may have atapered rather than flat upper surface. The shell may be made of anysuitable thermally insulating or thermally conductive material, such asmetal, ceramic, etc.

The stack(s) 9 and the shell 11 are removably positioned or removablyconnected to an upper surface (such as the base plate 7) of the base 3.Preferably, each fuel cell stack 9 and the shell 11 are separatelyremovably connected to the upper surface 7 of the base 3. In this case,the shell 11 may be easily removed from the upper surface 7 of the base3 without removing the stack(s) 9 under the shell 11. Alternatively, ifthe shell 11 contains a door or a hatch, then the stack(s) 9 under theshell 11 may be easily removed through the door or hatch withoutremoving the shell 11.

The term “removably connected” means that the stack(s) 9 and/or theshell 11 are connected to the upper surface 7 of the base 3 in such away as to be easily removed for repair or servicing. In other words,“removably connected” is an opposite of “permanently connected”. Forexample, the stacks 9 and/or the shell 11 are removably connected to theupper surface 7 of the base 3 by at least one of a snap fit connection,a tension fit connection, a fastening connection or a slide railconnection. An example of a snap fit connection is a bayonet typeconnection in which one or more prongs which hold a component in placeby hooking into an opening are pressed inward or outward to unhook themfrom the opening. An example of a tension fit connection is where acomponent, such as a stack 9 or a shell 11, is pressed into an openingor groove in the surface 7 of the base 3 which has the about same sizeas the cross section of the stack 9 or the shell 11 such that tensionholds the stack or the shell in the opening or groove. An example of afastening connection is connection by a fastener, such as a bolt or aclip, which can be removed by service personnel. An example of a sliderail connection is a drawer or dove tail type connection, such as agroove in the upper surface 7 of the base 3 into which a protrusion inthe stack 9 can be slid into, or a groove in the bottom stack 9 plateinto which a protrusion in the upper surface 7 of the base 3 can be slidinto. An example of a permanent connection is a welded connection, suchas where the shell 11 is welded to the surface 7 of the base.

The stack(s) 9 and the shell 11 can be removably connected using adifferent type of connection from each other. Furthermore, in analternative aspect of the invention, the shell 11 may be removablyconnected to the upper surface 7 of the base 3, while the stack(s) 9 maybe non-removably connected to the same surface 7.

Preferably, at least one heat exchanger is located in the interiorvolume 5 of the base 3. For example, as shown in FIG. 1B, a multi-streamheat exchanger 13 is located in the interior volume 5 of the base 3.

The heat exchanger 13 may comprise a low temperature portion 15 and ahigh temperature portion 17. The low temperature portion 15 may be madeof less expensive, low temperature materials, such as stainless steel,which are not tolerant of very high temperatures. The high temperatureportion 17 may be made of more expensive, high temperature materials,such as Inconel or other nickel alloys, which are high temperaturetolerant. This configuration decreases the cost of the heat exchanger13. If desired, one or more intermediate temperature portions made ofintermediate temperature tolerant materials may also be provided in theheat exchanger 13.

Any type of heat exchanger may be used, such as a finned plate type ofheat exchanger. If desired, the high temperature portion 17 of the heatexchanger may act as a complete or partial external reformer 37 for thefuel cell stacks 9. In this case, the all or a portion of fins of thepassages of the heat exchanger 13 which carry the fuel inlet stream arecoated with a fuel reformation catalyst, such as nickel and/or rhodiumfor a hydrocarbon fuel, such as natural gas or methane. The externalreformer 37 may act as a pre-reformer if the stacks 9 contain fuel cellsof the internal reformation type (i.e., fuel cells contain one or moreinternal surfaces or coatings that are catalytically active forreforming. The catalyst may comprise a catalyst coating, or using nickelas part of the metal construction of the fuel cell housing and support).Alternatively, for complete internal reformation type fuel cells or forfuel cell systems which operate on hydrogen fuel (which does not requirereformation), the reformer 37 may be omitted. For external reformationtype fuel cells (i.e., fuel cells which do not contain a fuelreformation catalyst or fuel cells in which the catalyst is part of themetal structure of the cell housing, the catalyst may still be present,but not designed to be used as a catalyst, usually due to degradation ofthe cells), the reformer 37 acts as the main fuel reformer. In analternative embodiment of the invention, the reformer 37 is notintegrated into the heat exchanger but is located in a separate locationin the hot box of the module 1. In another alternative embodiment of theinvention, separate fuel and air heat exchangers provide heat from thefuel and air exhaust streams, respectively, to fuel and air inletstreams, respectively, as will be described with respect to FIG. 2Bbelow.

As shown in FIGS. 1A-1E, an anode tail gas oxidizer (ATO) 10 ispreferably located over the central portion of the base 3 (i.e., on thebase plate 7) in a heat transfer relationship with the stacks 9 (i.e.,such that heat is transferred by convection and/or radiation between theATO 10 and the stacks 9). Preferably but not necessarily, the ATO 10 islocated in the middle of the annular stack 9 array such that the ATO 10is surrounded by the stacks 9. However, for stack 9 layouts that do notform a complete ring, such as grid or arc layouts, the ATO 10 may belocated adjacent to the stacks or may be partially surrounded by thestacks 9. In an annular or arc array, the ATO is exposed to the radiallyinward faces of the fuel cell stacks to receive the cathode exhauststream therefrom. An ATO is a chamber in which the anode (fuel) exhaustfrom the stacks is oxidized by reaction with an oxidizer stream, such asa reaction of the stack anode exhaust stream with the stack cathode(air) exhaust stream. The ATO chamber walls may be coated with asuitable oxidation reaction promoting catalyst, such as palladium on asupport member or material. The oxidation reaction releases heat whichcan be used to heat the stacks 9 and/or to provide a hot ATO exhauststream into the heat exchanger 13. As shown in FIG. 1B, the ATO 10 maycomprise an ATO exterior baffle 12, which is a cylindrical or othersuitably shaped wall which is attached to the top of the outer shell 11,but which contains an opening 18 adjacent to the base plate 7 of thebase 3 through which the stack cathode (air) exhaust stream passes. TheATO 10 may also comprise an interior baffle 14 which is a cylindrical orother suitably shaped wall which is attached to the base plate 7 butwhich contains an opening 20 adjacent to the upper surface of the shell11 through which the anode and cathode exhaust streams pass. Theinterior baffle 14 is preferably located inside the exterior baffle 12.The interior baffle 14 may also be considered as an annulus for theATO/cathode exhaust conduit 27. The interior and/or exterior surface ofthe interior baffle 14 and/or the interior surface of the exteriorbaffle 12 may be covered with the oxidation promoting catalyst material,which may be coated on optional fins or corrugations 16 located on thesurface(s) of the baffle(s) 12, 14. For example, while FIG. 1B shows atwo pass ATO (up flow, then down flow), the ATO 10 may have more passes,and the interior baffle 14 may contain perforations. Alternatively, theinterior baffle 14 may extend to the top of the shell 11 and only haveperforations rather than the opening 20 near the top.]

One or more ATO fuel inlet conduit(s) 22 are located in the base plate 7between the exterior 12 and the interior 14 ATO baffles. The ATO fuelinlet conduits 22 provide the ATO fuel inlet stream between the baffles12 and 14 where the fuel inlet stream mixes and reacts with the ATO airinlet stream. The ATO fuel inlet stream may comprise one or both of i) aseparate fuel inlet stream from the stack fuel inlet stream, such as anatural gas inlet stream, and/or ii) at least a portion of the stackanode exhaust stream that has passed through the heat exchanger 13.Alternatively, the ATO fuel inlet stream may also partially or fullybypass the heat exchanger to keep the inlet temperature limited. The ATOair inlet stream may comprise the stack cathode exhaust stream whichflows from the stacks 9 to the ATO 10 under the outer baffle 12, asshown in FIG. 1B, or a fresh air inlet stream (which may or may not bemixed with either of the ATO fuel inlet streams), or a combination offresh air inlet stream and stack cathode exhaust. The ATO fuel inletstream is oxidized by the ATO air inlet stream (such as the stackcathode exhaust stream or a mixture of the cathode exhaust and theoptional fresh air inlet streams]. The ATO exhaust stream (oxidizedfuel) is removed from the ATO 10 through the central ATO exhaust conduit27 located in the base plate 7 in the middle of the interior baffle 14.

As shown in FIGS. 1B and 1C, the base 3 also contains a plurality offuel inlets 21 which provide a fuel inlet stream to the fuel cell stacks9, a plurality of fuel exhaust openings 23 which remove the fuel exhauststream from the stacks 9, a plurality of peripheral air inlets 25 whichprovide an air (or other oxidizer) inlet stream to the stacks 9, and acentral ATO exhaust conduit 27 which removes the air/ATO exhaust streamfrom the stacks 9. Inlets 21 and 25 and exhaust opening 23 may compriseholes in the base plate 7 and/or pipes which extend through the baseplate 7. Thus, in one embodiment of the invention, the stacks 9 areexternally manifolded for air and internally manifolded for fuel. Theplurality of angularly spaced fuel cell stacks 9 are arranged to form anannular array about a central axis of the module inside the ring-shapedarrangement of the stack air inlets 25.

The module 1 operates as follows. The fuel and air inlet streams areheated in the heat exchanger 13 by the anode exhaust and/or the ATOexhaust streams, as will be described in more detail below. The fuelinlet stream is provided upwards and internally into the stacks 9through the respective fuel inlets 21 for each stack from below. Theanode (fuel) exhaust stream from the stacks 9 is provided downwards andinternally through the stacks and is removed through the respective fuelexhaust openings 23 into the heat exchanger 13 located in the base 3.

As shown by the arrows in FIG. 1B, the stack air inlet stream isprovided under the shell 11 through the base plate 7 through inlets 25arranged in an annular or ring shaped configuration in the periphery ofthe base 3. The air inlet stream flows through the cells of the stacks9. The stacks 9 and ceramic spacers (which are not shown for clarity)prevent the air inlet stream from flowing directly into the interiorspace 24 without flowing though the stacks 9 first. The cathode (air)exhaust stream exits the stacks 9 into the space 24 between the stacks 9and the outer ATO baffle 12. The cathode exhaust stream flows throughopening(s) 18 below the outer ATO baffle 12 into the space 26 betweenthe outer and inner ATO baffles 12, 14. The stack cathode exhaust streammixes and reacts with the ATO fuel inlet stream provided from conduits20 in space 26. The oxidation reaction provides heat to the stacks 9 viaradiation and/or convection during system start-up and during steadystate operation to provide sufficient heat for internal fuel reformationreaction in the stacks 9. The ATO exhaust (oxidized fuel) is thenexhausted upwards through opening(s) 20 above the inner baffle 14 anddownward through the central ATO exhaust conduit 27 into the heatexchanger 13 located in the chamber 5 below the base plate 7. While aspecific ATO configuration is shown in FIGS. 1B and 1C, it should beunderstood that other configurations may also be used, such asconfigurations where the fluid streams follow a linear or tortuous pathadjacent to oxidation catalyst coated members. For example, a cylindermay be located inside baffle 14 to limit the volume (and hence theamount) of fins and catalyst.

As shown in FIGS. 1A-1C, a fuel inlet line 29 is connected to a firstinlet of the fuel heat exchanger 13. The plurality of fuel inletconduits 21 are fluidly connected to a first outlet of the heatexchanger 13. The term “fluidly connected” means either directlyconnected or indirectly connected such that the fuel inlet stream flowsfrom the heat exchanger 13 through one or more other components until itreaches each fuel inlet conduit 21. The plurality of fuel exhaustopenings 23 are fluidly connected to a second inlet of the heatexchanger 13. A fuel exhaust line 31 is connected to a second outlet ofthe heat exchanger 13. An air inlet line 33 is connected to a thirdinlet of the heat exchanger 13. If desired, one or more optional airby-pass conduits may be provided which diverts some or all of the airinlet stream from the air inlet line 33 around the heat exchanger 13, oraround a portion of the heat exchanger 13. Thus, the by-pass conduit(s)may connect the air inlet line 33 directly to the stack 9 air inlet. Theamount of air provided into the by-pass conduit(s) can be controlled byflow regulator, such as a computer or operator controlled valve. Theplurality of air inlet conduits 25 in the base are fluidly connected toa third outlet of the heat exchanger 13. The central air/ATO exhaustconduit 27 is fluidly connected to a fourth inlet of the heat exchanger13. An air/ATO exhaust line 35 is connected to a fourth outlet of theheat exchanger 13. If desired, the heat exchanger 13 may have separateair and ATO exhaust lines (i.e., some or all of the hot air exhaust maybypass the ATO, which can instead use fresh inlet air for the oxidationreaction).

Preferably, the base 3 and the shell 11 are also used to provide anelectrical connection from the stacks 9 to the power conditioningequipment. For example, the upper surface 7 of the base 3 may contain aplurality of electrical contacts 41 such as negative or groundelectrical contacts. Each contact 41 is located where a bottom end plateof a fuel cell stack 9 would touch the base plate 7 (i.e., the uppersurface) of the base 3. Each negative or ground electrode or end plateof each fuel cell stack 9 is electrically connected to one of theplurality of electrical contacts 41. The base 3 also contains a commonelectrical bus 43, such as a negative or ground bus, which iselectrically connected to the fuel cells 9 through the contacts 41.

The shell 11 contains at least one other electrical bus 45, such as aseparate electrical bus 45 for each stack 9. The bus 45 has a differentpolarity than the polarity of the common electrical bus 43. For example,the shell 11 may have a plurality of positive buses 45. A positiveelectrode or end plate of a fuel cell stack 9 is electrically connectedto a respective positive electrical bus 45 extending from the shell 11.

The positive electrode or end plate of each fuel cell stack 9 may beelectrically connected to the respective positive electrical bus 45using any suitable contact or electrical connection. For example, asshown in FIG. 1B, an upper interior surface of the shell 11 contains aplurality of electrically conductive pressure members 47. The pressuremembers 47 on the shell 11 are aligned with the stack 9 positions overthe contacts 41 on the upper surface 7 of the base 3. Each pressuremember 47 removably holds at least one fuel cell stack 9 between theshell 11 and the upper surface 7 of the base 3. The positive electrodeor end plate of each fuel cell stack 9 is electrically connected to thepositive electrical bus 45 through a respective pressure member 47. Thepressure member 47 may be a flexible bar, plate or spring which puts adownward pressure on the stack 9 to keep the stack 9 firmly against theelectrical contact 41 on the upper surface 7 of the base. When the shell11 is pushed down to close the module 1, the pressure member flexes topress the stack 9 into place on the base 3. When the shell 11 is removedto service or repair the module, the pressure member releases the stack9.

Preferably, but not necessarily, each stack 9 or each pair of stacks 9are connected to a separate DC/DC converter unit of the powerconditioning system. For example, one electrical input/output of eachstack in each pair of stacks may be connected in series and the otherelectrical input/output of each stack in each pair of stacks provides arespective positive and negative voltage inputs into the respectiveDC/DC converter unit. Preferably, but not necessarily, the fuel cellstacks (i.e., fuel cell stack columns) may be arranged in a multiple ofsix to simplify power conditioning, as described in U.S. applicationSer. Nos. 11/797,707 and 11/707,708, filed on May 5, 2007 andincorporated herein by reference in their entirety. Thus, each modulemay have 6, 12, 18, 24, etc. stacks 9. For example, the module 1 shownin FIGS. 1C to 1E contains twelve stacks 9. Each set of four stacks maybe connected to one respective phase output of a three phase AC output,as described in U.S. application Ser. No. 11/797,707.

Thus, in a system comprising a plurality of modules, each module 1 maybe electrically disconnected, removed from the fuel cell system and/orserviced or repaired without stopping an operation of the other modules1 in the fuel cell system. In other words, each module 1 may beelectrically disconnected, removed from the fuel cell system and/orserviced or repaired while the other modules 1 continue to operate togenerate electricity. Thus, the entire fuel cell system does not have tobe shut down when one stack 9 malfunctions or is taken off line forservicing.

When one module 1 is taken off line (i.e., it is turned off to beremoved, repaired or serviced), while the other modules 1 continue tooperate, the flow of fuel to the module 1 which is taken off line shouldbe stopped. This may be accomplished by placing valve in each fuel inletline 29. The valve may be turned off manually or electronically to stopthe flow of fuel through a given fuel inlet line 29, while the fuelcontinues to flow through the other fuel inlet lines 29 to the othermodules 1.

The second embodiment of the invention provides a multi-stream heatexchanger 13 for a fuel cell system, where more than two fluid streamsexchange heat in the same device. Thus, a single multi-stream heatexchanger can replace multiple separate heat exchangers, such asseparate air and fuel heat exchangers, used in prior art systems. Themulti-stream heat exchanger allows for the same amount of heat exchangeas separate fuel and air heat exchangers, but with a smaller amount ofheat transfer area due to more uniform temperature differences betweenthe hot streams and cold streams. Furthermore, if desired, a steamgenerator and/or an external reformer 37 may be physically integratedinto the multi-stream heat exchanger 13 such that the heat of the fuelcell stack 9 anode exhaust stream and/or ATO 10 exhaust stream is usedto convert water to steam and/or to provide heat for a hydrocarbon fuelto hydrogen and carbon monoxide fuel reformation reaction, such as asteam-methane reformation (“SMR”) reaction.

The multi-stream heat exchanger 13 may serve as a base or be located inthe base 3 for building the hot box of the fuel cell system. Thus, themulti-stream heat exchanger 13 lowers the center of gravity of themodule 1 and makes the module more stable. The use of a singlemulti-stream heat exchanger 13 reduces the number of air flow controlsin the system from two to one. The ATO air flow control may beeliminated. It makes the system integration simpler by reducing theamount of additional plumbing. Furthermore, the multi-stream heatexchanger 13 increases the efficiency of the system, facilitating betterheat transfer, removing pinch points and reducing the parasitic losses,including the gain from the elimination of the ATO air blower. Finally,the multi-stream heat exchanger 13 allows the use of a combination oflow and high temperature materials in zones 15 and 17 to reduce the costof the device.

FIG. 2A illustrates a process flow diagram for a fuel cell system 100containing one or more modules 1 of the second embodiment. One module 1is shown for clarity in FIG. 2A. The system 100 contains the pluralityof the fuel cell stacks 9, such as a solid oxide fuel cell stacks (whereone solid oxide fuel cell of the stack contains a ceramic electrolyte,such as yttria stabilized zirconia (YSZ) or scandia stabilized zirconia(SSZ), an anode electrode, such as a nickel-YSZ or Ni—SSZ cermet, and acathode electrode, such as lanthanum strontium manganite (LSM)). Themodule 1 is represented as a hot box which may comprise the combinationof the base 3 and the shell 11, as shown in FIG. 1B. The optionalreformer 37 is shown separately from the heat exchanger 13. However, asnoted above, the heat exchanger 37 may be physically integrated into theheat exchanger 13.

The system 100 also contains a steam generator 103. The steam generator103 is provided with water through conduit 30A from a water source 104,such as a water tank or a water pipe, and converts the water to steam.The steam is provided from generator 103 to mixer 105 through conduit30B and is mixed with the stack anode (fuel) recycle stream in the mixer105. The mixer 105 may be located inside or outside the hot box of themodule 1. Preferably, the humidified anode exhaust stream is combinedwith the fuel inlet stream in the fuel inlet line or conduit 29downstream of the mixer 105, as schematically shown in FIG. 2A.Alternatively, if desired, the fuel inlet stream may also be provideddirectly into the mixer 105, or the steam may be provided directly intothe fuel inlet stream and/or the anode exhaust stream may be provideddirectly into the fuel inlet stream followed by humidification of thecombined fuel streams, as shown in FIGS. 1C, 1D and 1E.

The steam generator 103 may be heated by a separate heater and/or by thehot ATO exhaust stream which is passed in heat exchange relationshipwith the steam generator 103. If the steam generator 103 is physicallyincorporated into the heat exchanger 13, then the steam generator mayalso be heated by the anode exhaust stream in the heat exchanger. Thesteam generator 103 may be physically located in the hot box, such asinside the chamber 5 of the base 3. Alternatively, the steam generator103 may be located outside the hot box of the module 1. Thus, as shownin FIG. 1C, if the steam generator 103 is located in the hot box of themodule, then water is provided from the water source 104 through conduit30. If the steam generator 103 is located outside of the hot box of themodule, then steam is provided from the water source 104 through conduit30.

The system 100 also contains a splitter 107, an optional water trap 109and a catalytic partial pressure oxidation (CPOx) reactor 111. The watertrap 109 and drain are not required if the anode exhaust stream providedto the ATO 10 can be kept sufficiently hot to avoid condensation. Thesystem operates as follows. The inlet fuel stream, such as a hydrocarbonstream, for example natural gas, is provided into the fuel inlet conduit29 and through the CPOx reactor 111. During system start up, air is alsoprovided into the CPOx reactor 111 to catalytically partially oxidizethe fuel inlet stream. During steady state system operation, the airflow is turned off and the CPOx reactor acts as a fuel passage way inwhich the fuel is not partially oxidized. Thus, the system 100 maycomprise only one fuel inlet conduit which provides fuel in bothstart-up and steady state modes through the CPOx reactor 111. Thereforea separate fuel inlet conduit which, bypasses the CPOx reactor duringsteady state operation is not required.

The fuel inlet stream is provided into the multi-stream heat exchanger13 where its temperature is raised by heat exchange with the ATO exhauststream and the stack anode (fuel) exhaust streams. The fuel inlet streamis then optionally provided into the optional reformer 37 which may beintegrated into the heat exchanger 13 or be located in the hot boxseparately from the heat exchanger 13. The fuel inlet stream is reformedin the reformer via the SMR reaction and the reformed fuel inlet stream(which includes hydrogen, carbon monoxide, water vapor and unreformedmethane) is provided into the stacks 9 through the fuel inlets 21. Thefuel inlet stream travels upwards through the stacks through fuel inletrisers in the stacks 9 and is oxidized in the stacks 9 duringelectricity generation. The oxidized fuel (i.e., the anode or fuelexhaust stream) travels down the stacks 9 through the fuel exhaustrisers and is then exhausted from the stacks through the fuel exhaustopening 23 into the heat exchanger 13.

In the heat exchanger 13, the anode exhaust stream heats the fuel inletstream and the air inlet stream via heat exchange. The anode exhauststream is then provided via the fuel exhaust conduit 31 into a splitter107. A first portion of the anode exhaust stream is provided from thesplitter 107 into the water trap 109. In the water trap 109, the wateris removed from the anode exhaust stream and the removed water is storedor drained via drain 112. The remaining anode exhaust stream may beprovided from the water trap 109 into the ATO 10 via conduit 113. Theanode exhaust stream may be provided with fresh fuel, such as naturalgas from conduit 115 into the ATO 10 through fuel inlets 22 as acombined ATO fuel inlet stream.

A second portion of the anode exhaust stream is recycled from thesplitter 107 into the fuel inlet stream. For example, the second portionof the anode exhaust stream is recycled through conduit 117 by a blower(not shown in FIG. 2A) into the mixer 105. The anode exhaust stream ishumidified in the mixer 105 by mixing with the steam provided from thesteam generator 103. The humidified anode exhaust stream is thenprovided from the mixer 105 into the fuel inlet conduit 29 where itmixes with the fuel inlet stream. Providing water from the water tank104 to make steam is optional. All of the humidification for the freshfuel can be provided by anode recycle stream.

The air inlet stream is provided by a blower (not shown) from the airinlet conduit 33 into the heat exchanger 13. The blower may comprise thesingle air flow controller for the entire system. In the heat exchanger,the air inlet stream is heated by the ATO exhaust stream and the anodeexhaust stream via heat exchange. The heated air inlet stream is thenprovided into the module through the air inlets 25. The air passesthrough the stacks 9 into the ATO 10. In the ATO 10, the air exhauststream oxidizes the ATO fuel inlet stream to generate an ATO exhauststream. The ATO exhaust stream is exhausted through the ATO exhaustconduit 27 into the heat exchanger 13. The ATO exhaust stream heats thefuel and air inlet streams in the heat exchanger 13 via heat exchange.The ATO exhaust stream (which is still above room temperature) isprovided from the heat exchanger 13 to the steam generator 103 viaconduit 119. The heat from the ATO exhaust stream is used to convert thewater into steam via heat exchange in the steam generator 103. The ATOexhaust stream is then removed from the system via conduit 35. If thesteam generator 103 is physically integrated into the heat exchanger 13,then conduit 119 can be omitted and the steam generation takes place inthe heat exchanger 13. Thus, by controlling the air inlet blower output(i.e., power or speed), the magnitude (i.e., volume, pressure, speed,etc.) of air introduced into the system may be controlled. The cathode(air) exhaust stream is used as the ATO air inlet stream, thuseliminating the need for a separate ATO air inlet controller or blower.Furthermore, since the ATO exhaust stream is used to heat the air andfuel inlet streams, the control of the single air inlet stream inconduit 33 can be used to control the temperature of the stacks 9 andthe ATO 10. If the air by-pass conduit is present, then this conduitenhances the ability to control the stack 9 and ATO 10 temperature bycontrolling the amount of air provided into the heat exchanger 13compared to the amount of air provided directly into the stacks 9through the by-pass conduit.

FIGS. 3 and 4 illustrate the fluid flows though an exemplary five zoneheat exchanger 13. The zones are labeled Z1 to Z5 in FIG. 4. It shouldbe noted that the heat exchanger 13 may have less than five zones, suchas one to four zones or more than five zones, such as six to ten zones.The heat exchanger may be a counterflow, a co-flow or a combinationthereof heat exchanger type having a plate and fin or other suitableconfiguration. Furthermore, the order of fluid flow introduction and theflow stream temperatures described below are exemplary and may bechanged depending on the specific system configuration.

The cold air inlet stream enters zone 1 of the heat exchanger at aboutroom temperature from conduit 33 and is heated by the hot anode exhauststream. The anode exhaust stream gives up some of its heat and exits aswarm anode exhaust stream (at a temperature of about 100 C, for example)into conduit 31.

The warmed air inlet stream (at a temperature of about 100 C) isprovided from zone 1 into zone 2 of the heat exchanger. The relativelycold fuel inlet stream (which has been warmed to about 100 C by theaddition of the steam from the steam generator and of the recycled anodeexhaust stream from conduit 117) is also provided from conduit 29 intozone 2 of the heat exchanger. The air and fuel inlet streams are notmixed but flow through different respective channels in zone 2 separatedby the heat exchanger plates, or in separate channels of a single heatexchanger plate. The air and fuel inlet streams are heated by the hotanode exhaust stream in zone 2 via heat exchange across the heatexchanger plates.

The warmed air and fuel inlet streams (at a temperature of about 150 C)are provided into zone 3 of the heat exchanger 13. The hot anode exhauststream also first enters the heat exchanger 13 in zone 3 at atemperature of about 800 C. The air and fuel inlet streams are heated bythe hot anode exhaust stream and by the hot ATO exhaust stream in zone 3via heat exchange across the heat exchanger plates. The anode and ATOexhaust streams are not mixed but flow through different respectivechannels in zone 3 separated by the heat exchanger plates. Afterexchanging heat, the warm ATO exhaust stream exits the heat exchanger 13in zone 3 into conduit 119 at a temperature of about 300 C. The ATOexhaust stream is then used to generate steam in the steam generator103. As can be seen from FIGS. 3 and 4, zone 3 may be the largest orlongest zone of the heat exchanger 3 (i.e., the zone with the longestfluid flow channel length) where the fluid streams spend the longesttime of any zone in the heat exchanger.

The further warmed air and fuel inlet streams (at a temperature of about600 C) are provided into zone 4 of the heat exchanger 13. The air andfuel inlet streams are heated by the hot ATO exhaust stream in zone 4via heat exchange across the heat exchanger plates. The warmed up airinlet stream exits the heat exchanger 13 in zone 4 into conduits 25 at atemperature of about 650 C to be provided into the fuel cell stacks 9.

The further warmed fuel inlet stream (at a temperature of about 650 C)is provided into zone 5 of the heat exchanger 13. The ATO exhaust streamfirst enters the heat exchanger 13 in zone 5 from conduit 27 at atemperature of about 875 C. The fuel inlet stream is heated by the hotATO exhaust stream in zone 5 via heat exchange across the heat exchangerplates. The warmed up fuel inlet stream exits the heat exchanger 13 inzone 5 into conduits 21 at a temperature of about 750 C to be providedinto the fuel cell stacks 9 (and/or into the reformer 37 if a separatereformer is present).

As shown in FIG. 3, a gap due to an about 1% heat exchanger leak isassumed. Furthermore, as shown in FIG. 3, the hot streams (ATO and anodeexhaust streams) are maintained at about the same temperature as eachother in each zone where they are both present. Likewise, the coldstreams (air and fuel inlet streams) are maintained at about the sametemperature as each other in each zone where they are both present.Finally, the global pinch point is shown in FIG. 3 if the heat exchanger13 is designed based on pinch technology.

With respect to FIG. 1B, the low temperature portion 15 of the heatexchanger 13 may correspond to zones 1 and 2 (and optionally an adjacentportion of zone 3) shown in FIG. 4, while the high temperature portion17 of the heat exchanger 13 may correspond to zones 4 and 5 (andoptionally an adjacent portion of zone 3) shown in FIG. 4.

FIG. 2B illustrates a schematic of a system 200 according to anotherembodiment of the invention in which the single multi-stream heatexchanger 13 is replaced with separate heat exchangers. The commonlynumbered elements which are common to both system 100 of FIG. 2A andsystem 200 of FIG. 2B will not be described again for the sake ofbrevity. As shown in FIG. 2B, the multi-stream heat exchanger 13 isreplaced with a fuel heat exchanger 137, an air heat exchanger 203 andan optional air preheater heat exchanger 205.

As shown in FIG. 2B, the external reformer 37 may be omitted if the fuelcells or the fuel cell stack 9 contain internal fuel reformationcatalyst. Alternatively, the fuel heat exchanger 137 may contain thereformation catalyst in the fuel inlet portion of the heat exchanger. Inthis case, the heat exchanger 137 functions as both a heat exchanger anda reformer.

If desired, the water trap 109 may be omitted and the entire portion offuel exhaust stream provided from splitter 107 into conduit 113 may berecycled into the ATO 113.

Furthermore, the natural gas inlet conduit 115 into the ATO 10 may beomitted. Instead all of the fuel for the ATO 10 may be provided from thefuel cell stack 9 anode tail gas recycle conduit 113. For a thermallywell packaged system with internal fuel reformation, the introduction offresh fuel into the ATO 10 through conduit 115 may be omitted. Instead,the amount of fresh fuel provided to the stack 9 via conduit 29 iscontrolled or adjusted to control the heating up process. Theelimination of the separate fuel conduit to the ATO (and associated fuelblower) and the use of the stack cathode exhaust stream as the source ofoxidizer gas in the ATO 10 (instead of using a separate air inletconduit to provide fresh air into the ATO 10) reduces the complexity andcost of the fuel cell and control systems and method of operating thesystem. For example, control of the main air inlet stream in conduit 33may be used as the main control for the system temperature.

The system 200 shown in FIG. 2B operates similarly to the system 100shown in FIG. 2A. However, in the system 200, the air inlet stream inconduit 33 is first provided into the optional air preheater heatexchanger 205 where the air inlet stream is preheated by the fuel(anode) exhaust stream. The terms fuel exhaust and anode exhaust areused interchangeably herein with respect to solid oxide fuel cellstacks. The preheated air inlet stream is then provided into the airheat exchanger 203 where it is heated by the ATO 10 exhaust stream fromconduit 27. The ATO exhaust stream is then provided from the air heatexchanger 203 via conduit 119 to the steam generator 103. Thehydrocarbon fuel inlet stream is provided via the fuel inlet conduit 29into the fuel heat exchanger 137. The fuel inlet stream is then providedinto the fuel cell stack(s) 9 via conduit 21 where the fuel inlet streammay be reformed internally. Alternatively, a separate external reformer37 or an external reformer integrated into heat exchanger 137 may beused instead. The fuel exhaust stream is provided form the stack(s) 9into the fuel heat exchanger 137 via conduit 23A. The fuel exhauststream is then provided from the fuel heat exchanger 137 via conduit 23Binto the optional air preheater heat exchanger 205. The fuel exhauststream is then provided from the air preheater heat exchanger 205 viaconduit 31 into the splitter 107.

If desired, the reformer 37 and/or the steam generator 103 mayoptionally be integrated into the existing zones of the heat exchangeror they may be added as additional zones. For example, the reformercatalyst may be provided into the fuel inlet stream conduits in zones 3,4 and/or 5 to integrate the reformer 37 into the heat exchanger 13.

The steam generator 103 may be physically integrated with the heatexchanger by adding the steam generator as one or more extra zones tothe heat exchanger 13. FIG. 5 illustrates a process flow diagram for asystem 200 containing a steam generator which is integrated intomulti-stream heat exchanger 13/103. In the example of FIG. 5, the heatexchanger contains seven zones. However, a heat exchanger containingmore than or less than seven zones may be used. Other elements shown inFIG. 5 having the same numbers as elements in FIG. 2A have beendescribed above with respect to FIG. 2A and will not be described againwith respect to FIG. 5 for brevity. The exemplary temperatures in eachelement are shown in a circle above the element. It should be noted thatother suitable temperatures may be used.

The following table describes the hot and cold fluid flow streamspassing through each of the seven zones Z1 to Z7 of the integrated heatexchanger/steam generator 13/103:

Zone Cold Side Stream Hot Side Stream Z1 Water ANEXH Z2 Water, Air ANEXHZ3 Water, Air ANEXH, ATO-EXH Z4 Water, Air, Fuel-mix ANEXH, ATO-EXH Z5Air, Fuel-mix ANEXH, ATO-EXH Z6 Fuel-mix ANEXH, ATO-EXH Z7 Fuel-mixATO-EXH

In the table above, “water” corresponds to the water inlet stream fromthe water source 104 and conduit 30A, “air” corresponds to the air inletstream from conduit 33, “fuel-mix” corresponds to the humidified fuelinlet stream from conduit 29, “ANEXH” corresponds to the anode exhauststream from conduit 23 and ATO-EXH corresponds to the ATO exhaust streamfrom conduit 27. Thus, “water” is present in zones Z1 to Z4 (enters inZ1 and exits in Z4), “air” is present in zones Z2 to Z5 (enters in Z2and exits in Z5) and “fuel-mix” is present in zones Z4 to Z7 (enters inZ4 and exits in Z7). These cold side streams are heated by the “ANEXH”stream in zones Z1 to Z6 (enters in Z6 and exits in Z1) and by theATO-EXH stream in zones Z3 to Z7 (enters in Z7 and exits in Z3).

Thus, zone Z1 corresponds to the steam generator 103, zones Z2 to Z4correspond to a hybrid steam generator/heat exchanger, and zones Z5 toZ7 corresponds to the heat exchanger. Of course other heat exchanger andflow configurations may also be used. It should be noted that in FIG. 5,if a liquid hydrocarbon fuel is used, then the liquid fuel may beprovided into the steam generator together with the water to vaporizethe liquid fuel. An optional liquid fuel/water mixer 201 may be used tomix the liquid fuel and water. Furthermore, an optional ATO fuel/anodeexhaust mixer 203 may be used to mix the ATO fuel, such as natural gasin conduit 115, with the anode exhaust in conduit 113, prior toproviding the mixed fuel into the ATO inlet 22.

FIGS. 6 and 7 are non-limiting, three dimensional cut-away views of twotypes of multi-stream plate heat exchangers. In should be noted thatother heat exchanger configurations may be used. FIG. 6 shows a heatexchanger 300 configuration where two streams exchange heat in each zone(such as zones Z1 and Z5 shown in FIG. 4). For example, streams 301 and302 exchange heat in zone 304 and stream 301 and 302 exchange heat inzone 305. Each zone 304, 305 contains ribbed or finned heat exchangeplates 306. An inlet/outlet manifold 307 is located between the zones.

FIG. 7 shows another heat exchanger 310 configuration where two coldstreams R1 and R2 (such as the air and fuel inlet streams) exchange heatwith a hot water containing stream, such as the anode or ATO exhauststream (which corresponds to zones Z2 and Z4 shown in FIG. 4). Theseconfigurations can be easily extended to four fluid streams (such aszone 3 shown in FIG. 4). The heat exchanger 310 may be similar to theplate type heat exchanger 300 and contain heat exchanger plates 316.However, for example, each plate 316 may contain six openings 317 toaccommodate three inlets and three outlets of the three streams and thethree streams are provided in every third space between the parallelplates 316. The heat exchanger may be configured to handle more thanthree streams and may have different configurations other than parallelplate type configurations.

Another embodiment of the invention provides a modular design for theentire fuel cell system rather than just for the fuel cell stackmodules. The modular system design provides flexible installation andoperation. Modules allow scaling of installed generating capacity,reliable generation of power, flexibility of fuel processing, andflexibility of power output voltages and frequencies with a singledesign set. The modular design results in an “always on” unit with veryhigh availability and reliability. This design also provides an easymeans of scale up and meets specific requirements of customer'sinstallations. The modular design also allows the use of available fuelsand required voltages and frequencies which may vary by customer and/orby geographic region. Thus, in summary, since the fuel cell system isdesigned as a modular set, it can be installed to accommodate therequirements of different customers and the elements of the system areable to work in concert to achieve a very high system reliability andavailability.

FIG. 8 shows an exemplary configuration of the modular fuel cell system60. The system 60 includes the following elements. The system 60includes a plurality of fuel cell stack modules 61. These modules 61 aredevices which contain the components used for generating DC power from areadily reformed fuel stream.

In one aspect of the second embodiment, each fuel cell stack module 61is the same as the module 1 of the first embodiment. Thus, each module61 shown in FIG. 8 may comprise a base 3, a shell 11 and one or morefuel cell stacks 9, as shown in FIG. 1B. For example, for a hightemperature fuel cell system, such as a SOFC or a molten carbonate fuelcell system, each fuel cell stack module 61 is the same as the module 1of the first embodiment. In an alternative aspect of the secondembodiment, each module 61 may comprise one base 3 and a plurality offuel cell stacks 9 covered by a plurality of shells 11. Alternatively,each module 61 may have a different structure or configuration from themodules 1 of the first embodiment. For example, for low temperature fuelcell systems, such as PEM systems, each module 61 can be different fromthe module 1 of the first embodiment. Thus, the system of the secondembodiment is applicable to high and low temperature fuel cell stackmodules.

Each module 61 contains at least one fuel cell stack 9. Multiple fuelcell stack modules 61 may be installed in a clustered installation, suchas for example, in a single hot box 62. A failure of a single fuel cellstack module 61 results only in a slightly degraded output capacity orslightly degraded system efficiency because the remaining fuel cellstack modules 61 continue operation.

The system 60 also contains one or more fuel processing modules 63.These modules are devices which contain the components used forpre-processing of fuel so that it can be readily reformed. The fuelprocessing modules 61 may be designed to process different sets offuels. For example, a diesel fuel processing module, a natural gas fuelprocessing module, and an ethanol fuel processing module may beprovided. The processing modules 63 may processes at least one of thefollowing fuels selected from natural gas from a pipeline, compressednatural gas, propane, liquid petroleum gas, gasoline, diesel, homeheating oil, kerosene, JP-5, JP-8, aviation fuel, hydrogen, ammonia,ethanol, methanol, syn-gas, bio-gas, bio-diesel and other suitablehydrocarbon or hydrogen containing fuels. If desired, the reformer 37may be located in the fuel processing module 63. Alternatively, if it isdesirable to thermally integrate the reformer 37 with the fuel cellstack(s) 9, then the reformer(s) 37 may be located in the fuel cellstack module(s) 61. Furthermore, if internally reforming fuel cells areused, then the external reformer 37 may be omitted entirely.

The system 60 also contains one or more power conditioning modules 65.These modules 65 are devices which contain the components for convertingthe DC power to AC power, connecting to the grid, and managingtransients. The power conditioning modules 65 may be designed convertthe DC power from the fuel cell modules 61 to different AC voltages andfrequencies. Designs for 208V, 60 Hz; 480V, 60 Hz; 415V, 50 Hz and othercommon voltages and frequencies may be provided. For example, eachmodule 65 may contain a dedicated DC/DC converter unit for each pair ofstacks 9 in a fuel cell module 61 and a common DC/AC converter unit forthe plural DC/DC converter units of each module 65.

Each type of module 61, 63, 65 may be installed in or on a separatecontainer, such as a box, rack or platform. Thus, the containers may belocated separately from each other, and may be moved, repaired orserviced separately. For example, as shown in FIG. 8, the fuel cellstack modules 61 are located in a common hot box 62. The fuel processingmodule or modules 63 may be located in a separate box 67. The powerconditioning module or modules 65 may be located on a separate rack 69.

The fuel cell systems described herein may have other embodiments andconfigurations, as desired. Other components may be added if desired, asdescribed, for example, in U.S. application Ser. No. 10/300,021, filedon Nov. 20, 2002, in U.S. application Ser. No. 11/656,006 filed on Jan.22, 2007, in U.S. Provisional Application Ser. No. 60/461,190, filed onApr. 9, 2003, and in U.S. application Ser. No. 10/446,704, filed on May29, 2003 all incorporated herein by reference in their entirety.Furthermore, it should be understood that any system element or methodstep described in any embodiment and/or illustrated in any figure hereinmay also be used in systems and/or methods of other suitable embodimentsdescribed above, even if such use is not expressly described.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedescription was chosen in order to explain the principles of theinvention and its practical application. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

What is claimed is:
 1. A method of operating a fuel cell stack module,comprising: providing a fuel inlet stream and an air inlet stream to aplurality of fuel cell stacks, wherein the plurality of fuel cell stacksis an annular array surrounding a central axis with each of the fuelcell stacks having a stacking direction extending parallel to thecentral axis; providing an anode tail gas oxidizer (ATO) fuel inlet anda cathode exhaust stream from the plurality of fuel cell stacks into anATO, wherein the ATO is located in the middle of the annular array andin a heat transfer relationship with the plurality of fuel cell stacks;and providing an ATO exhaust stream from the ATO to heat the fuel inletstream and the air inlet stream via heat exchange between the ATOexhaust stream and the fuel and air inlet streams, wherein the heatexchange occurs across a surface between the ATO exhaust stream and eachof the fuel and air inlet streams.
 2. The method of claim 1, furthercomprising providing the ATO exhaust stream, the fuel inlet stream andthe air inlet stream into a multi-stream heat exchanger, wherein theheat exchange across the surface between the ATO exhaust stream and eachof the fuel and air inlet streams occurs within the multi-stream heatexchanger.
 3. The method of claim 2, wherein: at least one wall of theATO is coated with an oxidation promoting catalyst material; and theheat exchanger is located in an interior volume of a base supporting theATO and the annular array of fuel cell stacks.
 4. The method of claim 3,further comprising reforming the fuel inlet stream in the heatexchanger.
 5. The method of claim 3, further comprising providing awater stream into the heat exchanger, vaporizing the water stream intosteam using the ATO exhaust steam and providing the steam into the fuelinlet stream in addition to providing the ATO exhaust stream from theATO to heat the fuel inlet stream and the air inlet stream via heatexchange between the ATO exhaust stream and the fuel and air inletstreams.
 6. The method of claim 3, further comprising controlling atemperature of the plurality of the fuel cell stacks and a temperatureof the ATO by controlling the air inlet stream.
 7. The method of claim2, further comprising providing an anode exhaust stream from theplurality of fuel cell stacks into the multi-stream heat exchanger tofurther heat the fuel inlet stream and the air inlet stream using theanode exhaust stream.
 8. The method of claim 1, wherein the heatexchange across the surface between the ATO exhaust stream and each ofthe fuel and air inlet streams comprises the heat exchange between theATO exhaust stream and each of the fuel and air inlet streams whichoccurs across heat exchanger plates.